Wireless communication systems may include a multi-RAT UE having capability to access two or more radio access technologies (RATs) or communication systems. To access a specific RAT, connection to the specific RAT is established on the basis of a UE request and transmission/reception of data is performed. However, a multi-RAT UE cannot simultaneously access multiple RATs even if the multi-RAT UE has capability to access two or more RATs. That is, the multi-RAT UE cannot simultaneously transmit and receive data through different RATs even if the UE has multi-RAT capability.
Since the conventional multi-RAT scheme does not require interworking between a wireless LAN and a cellular network, system efficiency thereof is low. In addition, the conventional multi-RAT scheme supports only flow mobility/IP-flow mapping at a network level without control at a radio level even if a UE can simultaneously access multiple RATs, thereby enabling simultaneous access to multiple RATs. For this reason, the conventional scheme has been performed on the basis of a request of the UE without requiring control connection between an AP and a cellular network.
Yet, since the related art is hard to identify a precise situation of a network and performs UE-oriented RAT selection, there is a limit in increasing overall efficiency of the network. In particular, if a user equipment (UE) is unable to perform uplink transmission or downlink reception to a specific communication system among a plurality of communication systems or a UE is located at an area where the uplink transmission or the downlink reception is not available, it is necessary to change a method for the UE to perform initial attach. However, a study on the method has not been performed yet.